HEAT OR CALORIC. 121 



Water 1 in volume, and mercury 2,= always the arithmetical mean ; 

 e. g. 70 3 , if the extremes be 100 and 40; hence, in equal vol- 

 umes, water has twice the capacity of quicksilver. 



In equal weights, one pound of water at 100,-f-one pound of mer- 

 cury at 40-~97j; therefore the 2| lost by the water have raised 

 the mercury 57 J, which is in the proportion of 1 : 23, viz, water has 

 twenty three times the specific heat that is contained in an equal 

 weight of mercury, and its capacity is in the same proportion.* 



(g.) Formula. 1 . By weight. If the weight be multiplied into the 

 change of temperature, the capacity will be inversely as the change, 

 that is, the greater the change, the less the capacity, and vice versa. 

 2. By volume ; the capacity found as above, X into the sp. gr. = the 

 capacity by volume. f 



(h.) Comparing classes of bodies, the capacities for heat are, in 

 general, inversely as their density. Solids have less capacity than 

 fluids fluids less than gases, and vice versa. 



When the capacity is enlarged, heat is absorbed, and when dimin- 

 ished, it is given out. 



ft;) The sudden expansion of air always produces cold. "This 

 striking occurrence takes place on a vast scale at the fountain of Hiero ; 

 at the mines of Chemnitz, 'in Hungary. A part of the machinery for 

 working these mines, is a perpendicular column of water, two hun- 

 dred and sixty feet high, which presses on a quantity of air enclosed 

 in a tight reservoir. The air is consequently condensed to an enor- 

 mous degree by this height of water, which is equal to eight or nine 

 atmospheres, and when a pipe, communicating with this reservoir of 

 condensed air, is suddenly opened, it rushes out with extreme ve- 

 locity, instantly expands, and in so doing absorbs so much caloric, as 

 to precipitate the moisture it contains in a shower of very white com- 

 pact snow, or rather hail, which may be readily gathered on a hat, 

 held in the blast. The force of this is so great, that the workman 

 who holds the hat is obliged to lean his back against the wall to re- 

 tain it in its position. If the cock of the pipe is only partly opened, 

 the snow is still more compact."{ 



By condensing aeriform bodies into a small space, cooling them 

 by freezing mixtures, and liberating them suddenly, great cold is 

 produced by the rarefaction. 



We have found occasion more than once to remark that similar 

 effects probably happen in the higher regions of the atmosphere, from 

 the sudden liberation of the ascending currents of rarefied air from 

 pressure, and from their mixture with colder currents. 



(j.) Great and sudden increase of pressure upon common air, evolves 

 so much heat as to ignite very combustible bodies. This was exhibit- 



* Henry's Chemistry. The author quotes Dalton ; the numbers usually stated 

 are as 1 to 28. f Murray. 1 Aikin, I, 213. 



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